The growth in use of small electrically-powered devices has increased the demand for very small metal-air electrochemical cells. Such small cells are usually disc-like or pellet-like in appearance, and are about the size of garment buttons. These cells generally have diameters ranging from less than 0.25 inch up to about 1.0 inch, and height ranging from less than 0.15 inch up to about 0.60 inch. The small size, and the limited amount of electrochemically reactive material which can be contained in these small metal-air cells result in considerable attention being directed to improving the efficiency and completeness of the power generating electrochemical reactions which occur therein, as well as to increasing the amount of reactive material which can be placed into the cell.
In general, metal-air cells convert atmospheric oxygen to hydroxyl ions in the air cathode of the cell. The hydroxyl ions then migrate to the anode, where they cause the metal contained in the anode to oxidize. Usually the active anode material in such cells comprises zinc.
More particularly, the desired reaction at a metal-air cell air cathode involves the reduction of oxygen, the consumption of electrons, and the production of hydroxyl ions, the hydroxyl ions being free to then migrate through the electrolyte toward the anode, where oxidation of zinc may occur, forming zinc oxide.
In most metal-air cells, air enters the cell through one or more ports in the cell. The ports extend through the bottom of the cathode can, and may be immediately adjacent the cathode assembly, or may be separated from the cathode assembly by an air chamber or an air diffusion member.
In any such arrangements, the port facilitates the movement of air into the cathode assembly. At the cathode assembly, the oxygen in the air reacts with water, as a chemically reactive participant in the electrochemical reaction of the cell, and thereby forms the hydroxyl ions.
In order for the electrochemical cell to survive normal conditions encountered in fabrication and use of the cell, the respective structural components of the cell must be able to withstand the normal conditions of fabrication and use, both individually and in combination. The main structural components are the anode can and the cathode can, plus an intervening seal which provides support between the anode can and the cathode can.
In general, the size of any given cell is limited by the inside dimensions of the space provided in the article in which the cell will operate. For example, the size of a hearing aid cell is limited to the internal dimensions of the cavity in the hearing aid appliance. The internal dimensions of the cavity are determined by the hearing aid manufacturer, not the power cell manufacturer.
Thus, any given appliance includes a limited amount of gross space/volume allotted to occupancy by the electrochemical cell which powers the appliance. That gross space is ultimately divided according to three competing, but supportive functions. A first and minimal portion of the gross space is used to provide clearance between the interior elements of the space and the exterior elements of the electrochemical cell. A second portion of the gross space is occupied by the structural and otherwise non-reactive elements of the electrochemical cell. The third portion of the gross space is occupied by the electrochemically reactive materials of the electrochemical cell.
Since the overall electrochemical capacity of any electrochemical cell is to some extent determined by the quantity of electrochemically reactive materials which can be loaded into the cell, it is important to maximize the volume of the space devoted to containing the reactive materials. It is correspondingly important to minimize the portions of the space that are used for providing clearance for the cell, and for providing structural support and other non-reactive elements within the cell.
Normal conditions of fabrication of the cell place significant structural stresses on both the anode can and the cathode can. Specifically, when the anode can and the cathode can are assembled to each other, a force pushing the cathode can toward the anode can may be used to correspondingly crimp the distal edge of the cathode can sidewall against the anode can and the intervening seal. An opposing force may be exerted on the anode can as part of the assembly process.
The physical properties of the structural elements of the cell, namely the anode can and the cathode can, must be strong enough to withstand especially the opposing forces used in assembling and closing the cell. Thus, the respective enclosing top, bottom, and side walls of the anode can and the cathode can must be strong enough to tolerate the assembly process without collapsing. Using conventional can structures and closure processes has, prior to this invention, suggested that the thickness of the can side walls be of the order of at least 0.008 inch in order for the cans to predictably tolerate the assembly process. If, however, the thickness of the side walls could be reduced, that would release additional internal volume in the cell for use in holding the electrochemically reactive material, e.g. metal anode material.
A further problem experienced with electrochemical cells of the "button-type" construction is that the bottom of the cathode can tends to become dished-in/concave during closing of the cell at final assembly. This problem is related to thickness of the metal used for fabricating the cathode can, and becomes more acute as one reduces the thickness of the metal.
It is an object of this invention to provide improved electrode can structure, especially cathode can structure, for an electrochemical cell by providing improved structure of the can at the corner joining the bottom of the can to a corresponding sidewall of the can.
It is another object to provide improved cathode can structure for a metal-air electrochemical cell, by providing improved structure of the can at the corner joining the bottom of the can to a corresponding side wall of the can.
It is yet another object to provide an electrode can having a bottom, and a side wall extending upwardly from the bottom, wherein force can be applied upwardly on the flat surface of the bottom of the can, and can be transmitted away from the bottom in a direct line through a side wall of the can and parallel to an inner surface of the side wall.
It is yet another object to provide an electrochemical cell wherein the cathode can, the anode can, or both, include improved structure of the respective can at the corner joining the closed end of the respective can to side wall of the respective can.
It is still another object to provide improved methods for forming the corner between the closed end of the respective can and the corresponding side wall of the can.
It is a further object to provide improved methods for assembling an anode, including an anode can, to a cathode, including a cathode can, to thus make a button-type electrochemical cell, including closing the cell so made by crimping the distal edge of the cathode side wall against the anode can and an intervening seal, while exerting a limited opposing force on the anode can.